Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users sequentially or simultaneously by sharing the available system resources. Examples of such systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, and Orthogonal Frequency Division Multiple Access (OFDMA) systems, or any combinations of these.
In wireless communication systems employing orthogonal frequency division multiplexing (OFDM), a transmitter transmits data to a receiver using many subcarriers in parallel. The frequencies of the sub-carriers are orthogonal. Transmitting the data in parallel allows the symbols containing the data to be of longer duration, which reduces the effects of multi-path fading. The orthogonality of the frequencies allows the sub-carriers to be tightly spaced, while minimizing inter-carrier interference. At the transmitter, the data is encoded, interleaved, and modulated to form data symbols. Each OFDM symbol is allocated to represent a component of a different orthogonal frequency. An inverse Fast Fourier Transform (IFFT) is applied to the OFDM symbol to generate time samples of a signal. Cyclic extensions, in particular in form of a cyclic prefix (CP), are added to the signal, and the signal is passed through a digital-to-analog converter. Finally, the transmitter transmits the signal to the receiver along a channel
When the receiver receives the signal, the inverse operations are performed. The received signal is passed through an analog-to-digital converter, and timing information is then determined. The cyclic extensions are removed from the signal. The receiver performs a fast Fourier transformation (FFT) on the received signal to recover the frequency components of the signal, that is, the data symbols. Error correction may be applied to the data symbols to compensate for variations in phase and amplitude caused during propagation of the signal along the channel. The data symbols are then demodulated, de-interleaved, and decoded, to yield the transmitted data.
The variations in phase and amplitude resulting from propagation along the channel are referred to as the channel response. The channel response is usually frequency and time dependent. If the receiver can determine the channel response, the received signal can be corrected to compensate for the channel degradation. The determination of the channel response is called channel estimation. OFDM systems promise high data rates with low complexity due to the simplicity of the FFT and one-tap propagation channel. However, this can be assured only if the receiver is in a synchronization mode. A drawback of OFDM systems is that they are vulnerable to frequency errors.
An accurate estimate of the response of a wireless channel between a transmitter and a receiver is needed in order to effectively transmit data on the available sub-bands. Channel estimation is typically performed by sending a pilot from the transmitter and measuring the pilot at the receiver. Since the pilot is made up of symbols that are known a priori by the receiver, the channel response can be estimated as the relation of the received pilot symbols over the transmitted pilot symbols. This relation may include any channel dependent information as amplitude, phase, frequency shift, angular spread, interference, noise, etc. The receiver compares the received value of the pilot symbols with the known transmitted value of the pilot symbols to estimate the channel response.
A pilot transmission represents an overhead in a wireless communication system. Thus, it is desirable to minimize pilot transmission to the extent possible. However, because of noise, fading, Doppler, interference, angular dispersion and other artifacts in the wireless channel, a sufficient amount of pilot energy needs to be transmitted frequently enough in order for the receiver to obtain a reasonably accurate estimate of the time-variant channel response. Because the contributions of physical scatters and the propagation paths to the channel response vary over time, the pilot transmission needs to be repeated regularly. The time duration over which the wireless channel may be assumed to be relatively constant is often referred to as a channel coherence time. The repeated pilot transmissions need to be spaced significantly closer in time than the channel coherence time to maintain high system performance. Similarly, for a wideband transmission, the pilot spacing in frequency also has to be sufficiently tight, to be able to estimate the full frequency dependent channel, which possibly extends over the coherence band. The coherence time of a channel may depend, for instance, on the velocity of the receiver. The coherence bandwidth of a channel may depend, for instance, on the delay spread of the channel.
In the downlink of a wireless communication system, a single pilot transmission from an access point (or a network element or a base station or a base station controller) may be used by a number of terminals to estimate the response of the distinct channels from the access point to each of the terminals. Furthermore, the pilot signals of different access points need to be separable form each other, from random data and from noise or interference to allow reliable estimation of the channel between the access point and the terminal. In the uplink, the channel from each of the terminals to the access point typically needs to be estimated through separate pilot transmissions from each of the terminals.
In the current IEEE 802.16m system, OFDMA is used in both uplink and downlink channels as the access technique. However, the single-carrier frequency division multiple access (SC-FDMA) with cyclic prefix (CP) provides additional advantages. It is well known that, compared to OFDMA, one of the main benefits provided by single-carrier transmission is the significantly lower peak-to-average power ratio (PAPR) or cubic metric (CM). The reduction of PAPR or CM channel provides corresponding improvements in power-amplifier efficiency and coverage area. Another problem with OFDMA in mobile environment results from an inevitable frequency offset in the frequency references among different terminals. It has already been demonstrated that using SC-FDMA can overcome this disadvantage, too. Therefore, there is an approach to incorporate the SC-FDMA concept into the IEEE 802.16m system.